1. Field of the Invention
The present invention relates to a signal light monitoring apparatus and a signal light monitoring method of judging whether signal light is present or absent in optical communication.
2. Description of Related Art
In optical communication, one method of judging whether signal light is present or absent is a method of judging whether signal light is present or absent based on whether a communication apparatus is successful at restoring data from input light.
For example, a communication apparatus (a light reception apparatus) that limits a band of input signal light, converts the band-limited signal light into an electrical signal, and performs an equalization process on the electrical signal is disclosed in a Pamphlet of International Publication No 2004/095740 (Patent Document 1). Since this communication apparatus limits the band of the input signal light, the communication apparatus can restore communication data even when an S/N ratio (Signal-To-Noise Ratio) is relatively small. When the communication apparatus is successful at restoring the communication data, the communication apparatus judges that the communication apparatus is receiving signal light.
However, the communication apparatus in Patent Document 1 does not have a judgment means for judging whether the communication apparatus is successful at restoring data, and cannot judge whether the communication apparatus is successful at restoring data or whether signal light is present or absent. Thereby, it is necessary for a user to judge whether the communication apparatus is successful at restoring data and whether the signal light is present or absent.
In order to restore the communication data, an S/N ratio equal to or more than a certain value so that a code error is not generated is necessary. Thereby, the communication apparatus in Patent Document 1 cannot restore the communication data and the user cannot accurately judge whether the signal light is present or absent in an optically noisy environment. Further, a degree of limitation of the band of the input light in the communication apparatus in Patent Document 1 is limited to a degree allowing a communication symbol to be decoded from signal light. Accordingly, improvement of the S/N ratio using the band limitation is limited. In this point, the communication apparatus in Patent Document 1 cannot accurately judge whether signal light is present or absent in an optically noisy environment.